Capacitors are widely used in integrated circuits. One of the most commonly used capacitors is the metal-insulator-metal (MIM) capacitor. FIG. 1 illustrates a typical MIM capacitor, which includes bottom plate 2, top plate 6, and insulation layer 4 therebetween. Bottom plate 2 and top plate 6 are formed of conductive materials.
As is known in the art, the capacitance of a capacitor is proportional to its area and the dielectric constant (k) of the insulation layer, and is inversely proportional to the thickness of the insulation layer. Therefore, to increase the capacitance, it is preferable to increase the area and k value and to reduce the thickness of the insulation layer. However, the thickness and k value are often limited by the technology used for forming the capacitor. For example, the thickness of the insulation layer is limited by the breakdown voltage. On the other hand, since the MIM capacitors are often formed in low-k dielectric layers, the ability to increase the k value is also limited.
Methods for increasing the area of the capacitor have also been explored. A problem associated with increased area is that greater chip area is required. This dilemma is solved by the introduction of vertical (multi-layer) capacitors, often referred to as metal-oxide-metal (MOM) capacitors. A typical vertical MOM capacitor 10 is shown in FIG. 2, which is a perspective view. Capacitor 10 includes metal electrodes 12 and 14 separated by dielectric materials 18. Each of the metal electrodes 12 and 14 forms a three-dimensional structure. For clarity, metal electrode 12 is shown as un-patterned, and metal electrode 14 is patterned with dots.
Each of the metal electrodes 12 and 14 includes more than one layer connected by vias, and each layer is formed in a metallization layer commonly used for the formation of interconnect structures. In addition to the capacitance in each of the metallization layers, the capacitance of capacitor 10 also includes portions created by the overlap between the different layers, and the overlapped portions contribute to the total capacitance of capacitor 10.
It is realized that different integrated circuits may have different requirements for the design of MOM capacitors. For example, some integrated circuits require the MOM capacitors to occupy as small a chip area as possible and are less demanding in the capacitances of the MOM capacitors. Conversely, other integrated circuits demand great capacitances and are relatively less demanding in chip area usage. The manufacturing cost is also an issue to be considered. New MOM structures to suit different requirements are thus needed.